The present invention relates to a method and device for magnetically moving and removing finely divided magnetic particles which are accumulated on or adhere to the walls of a pipe or the like in such a way that they will not be entrained by a fluid flowing through the pipe or the like.
For instance, CRUD (Canadian Reactor Unknown Deposite), which refers to finely divided particles of radioactive iron scales, is produced from the pipes, fuel rods and so on in a nuclear reactor and is deposited in pipes and tanks in primary and secondary cooling systems. CRUD consists of finely divided particles of ferromagnetic iron oxides such as magnetite (Fe.sub.3 O.sub.4) and magnetic (gamma-Fe.sub.2 O.sub.3) and of paramagnetic iron oxide such as hematite (alpha-Fe.sub.2 O.sub.3). These particles are highly contaminated with radioactivity so that there is a fear that the operators who make periodic inspection, maintenance and repairs of the equipment at or near the places where such radioactive waste is accumulated are damaged or injured by radiation. CRUD tends to accumulate at such places in the pipes and tanks where the liquid flows slowly. For instance, as will be described below, CRUD accumulates in large quantities in the space between the safe-end nozzle and thermal sleeve. As a result, it is very dangerous to make access to these component parts for periodic inspection, maintenance and repairs and, consequently, a working time per day of one operator is determined by a tolerable biological dose rate and is very short in practice. It follows therefore that it takes many man-hours to complete the periodic inspection, maintenance and repairs of one reactor with the resultant increase not only in period but also in cost. More importantly, it should be avoided to let the operators to work at such hazardous places.
However, so far there have never been taken any positive and effective countermeasures against CRUD accumulation or deposition mainly because of the fact that CRUD is not susceptible to magnetism. The only countermeasure so far taken is to use the lead radiation shielding when the operators approaches the safe-end nozzles or the like as will be described below.
Referring to FIG. 1, in the case of the periodic inspection of a safe-end nozzle b extended from the wall of a pressure vessel a of a nuclear reactor, ring-shaped lead shielding bodies d are inserted into a thermal sleeve c and fitted over the safe-end nozzle b. Thereafter the opening of a biological shiled e is completely sealed with packs f filled with lead particles. Then the biological dose rate due to the pressure of CRUD in the space g between the safe-end nozzle b and the thermal sleeve c can be reduced.
In the case of the nuclear reactor plants, it is difficult to cut a pipe or the like at desired points and therefore the transport and removal of CRUD by the manipulation of a suitable gas or the like or a mechanical device from the exterior of the pressure vessel a is almost impossible. The above-described method for shielding with the formed lead shields d and the packs f of lead particles involves the following problems:
(1) The biological dose rate is too high for the operators. PA1 (2) The formed lead shields d and the lead-particle-filled packs f are heavy so that their handling is dangerous and many handling steps are involved. PA1 (3) A long time is required for shielding. As a result, it is impossible to employ this shielding arrangement when the periodic inspection, maintenance and reparis schedule is very tight. PA1 (4) The cost for the periodic inspection, maintenance and repairs becomes high because of the cost for installing and removing the shields d and the packs f. PA1 (5) The shields d in predetermined shape can be used so that they cannot be used in other places. In addition, after the periodic inspection, maintenance and repairs, these shields d and packs f must be stored for a next operation and therefore a storage space is required. PA1 (6)Shielding is provided to reduce the biological dose rate as little as possible so that the formed lead shields d and the packs f filled with lead particles are considerably large in size, especially in thickness. As a result, the working space left is limited so that the periodic inspection, maintenance and repairs becomes difficult to accomplish. PA1 (1) A method in which, as in a linear motor, there is produced three-phase AC, flat moving fields. PA1 (2) A method in which the alternating field produced by the alternative or DC electromagnet or permanent magnet is displaced mechanically. PA1 (3) A method in which the DC field produced by the DC or permanent magnet is displaced mechanically. However, by any of the above three methods, the transportation facility of CRUD is 10% at the highest. In addition, a wide working space is needed.
In the nuclear power plants, in view of the safety problems of the component parts and because of the laws, orders, rules and the like regarding the safety of the operation of the nuclear reactor, it is next to impossible to cut a pipe or the like at desired points so that the jet of air or the like may be forced into the space.
In order to solve the problem of removing CRUD especially from the space g between the safe-end nozzle b and the thermal sleeve c, the inventors made extensive studies and experiments and found out that in the accumulation of CRUD the hematite particles, which are essentially non-magnetic, are surrounded by the magnetite and maghemite particles which are magnetic. Based upon this observed fact the inventors have proposed a novel method and device for magnetically moving and removing CRUD.
There have been proposed three types of magnetically removing CRUD as described below:
In view of the above, the primary object of the present invention is to provide a method and device for magnetically transporting or removing magnetic particles especially CRUD with a higher degree of efficiency. The present invention will become apparent from the following description of one preferred embodiment thereof taken in conjunction with the accompanying drawings.